Morphology is the identification, analysis and description of structure of words (words as units in the lexicon are the subject matter of lexicology). While words are generally accepted as being (with clitics) the smallest units of syntax, it is clear that in most (if not all) languages, words can be related to other words by rules. For example, English speakers recognize that the words dog, dogs, and dog catcher are closely related. English speakers recognize these relations from their tacit knowledge of the rules of word formation in English. They infer intuitively that dog is to dogs as cat is to cats; similarly, dog is to dog catcher as dish is to dishwasher. The rules understood by the speaker reflect specific patterns (or regularities) in the way words are formed from smaller units and how those smaller units interact in speech. In this way, morphology is the branch of linguistics that studies patterns of word formation within and across languages, and attempts to formulate rules that model the knowledge of the speakers of those languages.
The history of morphological analysis dates back to the ancient Indian linguist, who formulated the 3,959 rules of Sanskrit morphology in the text by using a Constituency Grammar. The Greco-Roman grammatical tradition also engaged in morphological analysis. Studies in Arabic morphology, conducted by and, date back to at least the 1200 CE.
The distinction between these two senses of "word" is arguably the most important one in morphology. The first sense of "word", the one in which dog and dogs are "the same word", is called a lexeme. The second sense is called word form. We thus say that dog and dogs are different forms of the same lexeme. Dog and dog catcher, on the other hand, are different lexemes, as they refer to two different kinds of entities. The form of a word that is chosen conventionally to represent the canonical form of a word is called a lemma, or citation form.
Here are examples from other languages of the failure of a single phonological word to coincide with a single morphological word form. In Latin, one way to express the concept of 'NOUN-PHRASE1 and NOUN-PHRASE2' (as in "apples and oranges") is to suffix '-que' to the second noun phrase: "apples oranges-and", as it were. An extreme level of this theoretical quandary posed by some phonological words is provided by the Kwak'wala language. In Kwak'wala, as in a great many other languages, meaning relations between nouns, including possession and "semantic case", are formulated by affixes instead of by independent "words". The three word English phrase, "with his club", where 'with' identifies its dependent noun phrase as an instrument and 'his' denotes a possession relation, would consist of two words or even just one word in many languages. But affixation for semantic relations in Kwak'wala differs dramatically (from the viewpoint of those whose language is not Kwak'wala) from such affixation in other languages for this reason: the affixes phonologically attach not to the lexeme they pertain to semantically, but to the preceding lexeme. Consider the following example (in Kwakw'ala, sentences begin with what corresponds to an English verb):
kwixʔid-i-da bəgwanəmai-χ-a q'asa-s-isi t'alwagwayu
Morpheme by morpheme translation:
kwixʔid-i-da = clubbed-PIVOT-DETERMINER
bəgwanəma-χ-a = man-ACCUSATIVE-DETERMINER
q'asa-s-is = otter-INSTRUMENTAL-3.PERSON.SINGULAR-POSSESSIVE
t'alwagwayu = club.
"the man clubbed the otter with his club"
That is, to the speaker of Kwak'wala, the sentence does not contain the "words" 'him-the-otter' or 'with-his-club' Instead, the markers -i-da (PIVOT-'the'), referring to man, attaches not to bəgwanəma ('man'), but instead to the "verb"; the markers -χ-a (ACCUSATIVE-'the'), referring to otter, attach to bəgwanəma instead of to q'asa ('otter'), etc. To summarize differently: a speaker of Kwak'wala does not perceive the sentence to consist of these phonological words:
kwixʔid i-da-bəgwanəma χ-a-q'asa s-isi-t'alwagwayu
"clubbed PIVOT-the-mani hit-the-otter with-hisi-club
A central publication on this topic is the recent volume edited by Dixon and Aikhenvald (2007), examining the mismatch between prosodic-phonological and grammatical definitions of "word" in various Amazonian, Australian Aboriginal, Caucasian, Eskimo, Indo-European, Native North American, West African, and sign languages. Apparently, a wide variety of languages make use of the hybrid linguistic unit clitic, possessing the grammatical features of independent words but the prosodic-phonological lack of freedom of bound morphemes. The intermediate status of clitics poses a considerable challenge to linguistic theory.
Given the notion of a lexeme, it is possible to distinguish two kinds of morphological rules. Some morphological rules relate to different forms of the same lexeme; while other rules relate to different lexemes. Rules of the first kind are called inflectional rules, while those of the second kind are called word formation. The English plural, as illustrated by dog and dogs, is an inflectional rule; compounds like dog catcher or dishwasher provide an example of a word formation rule. Informally, word formation rules form "new words" (that is, new lexemes), while inflection rules yield variant forms of the "same" word (lexeme).
There is a further distinction between two kinds of word formation: derivation and compounding. Compounding is a process of word formation that involves combining complete word forms into a single compound form; dog catcher is therefore a compound, because both dog and catcher are complete word forms in their own right before the compounding process has been applied, and are subsequently treated as one form. Derivation involves affixing bound (non-independent) forms to existing lexemes, whereby the addition of the affix derives a new lexeme. One example of derivation is clear in this case: the word independent is derived from the word dependent by prefixing it with the derivational prefix in-, while dependent itself is derived from the verb depend.
The distinction between inflection and word formation is not at all clear cut. There are many examples where linguists fail to agree whether a given rule is inflection or word formation. The next section will attempt to clarify this distinction.
A linguistic paradigm is the complete set of related word forms associated with a given lexeme. The familiar examples of paradigms are the conjugations of verbs, and the declensions of nouns. Accordingly, the word forms of a lexeme may be arranged conveniently into tables, by classifying them according to shared inflectional categories such as tense, aspect, mood, number, gender or case. For example, the personal pronouns in English can be organized into tables, using the categories of person (1st., 2nd., 3rd.), number (singular vs. plural), gender (masculine, feminine, neuter), and case (subjective, objective, and possessive). See English personal pronouns for the details.
The inflectional categories used to group word forms into paradigms cannot be chosen arbitrarily; they must be categories that are relevant to stating the syntactic rules of the language. For example, person and number are categories that can be used to define paradigms in English, because English has grammatical agreement rules that require the verb in a sentence to appear in an inflectional form that matches the person and number of the subject. In other words, the syntactic rules of English care about the difference between dog and dogs, because the choice between these two forms determines which form of the verb is to be used. In contrast, however, no syntactic rule of English cares about the difference between dog and dog catcher, or dependent and independent. The first two are just nouns, and the second two just adjectives, and they generally behave like any other noun or adjective behaves.
An important difference between inflection and word formation is that inflected word forms of lexemes are organized into paradigms, which are defined by the requirements of syntactic rules, whereas the rules of word formation are not restricted by any corresponding requirements of syntax. Inflection is therefore said to be relevant to syntax, and word formation is not. The part of morphology that covers the relationship between syntax and morphology is called morphosyntax, and it concerns itself with inflection and paradigms, but not with word formation or compounding.
In the exposition above, morphological rules are described as analogies between word forms: dog is to dogs as cat is to cats, and as dish is to dishes. In this case, the analogy applies both to the form of the words and to their meaning: in each pair, the first word means "one of X", while the second "two or more of X", and the difference is always the plural form -s affixed to the second word, signaling the key distinction between singular and plural entities.
One of the largest sources of complexity in morphology is that this one-to-one correspondence between meaning and form scarcely applies to every case in the language. In English, we have word form pairs like ox/oxen, goose/geese, and sheep/sheep, where the difference between the singular and the plural is signaled in a way that departs from the regular pattern, or is not signaled at all. Even cases considered "regular", with the final -s, are not so simple; the -s in dogs is not pronounced the same way as the -s in cats, and in a plural like dishes, an "extra" vowel appears before the -s. These cases, where the same distinction is effected by alternative forms of a "word", are called allomorphy.
Phonological rules constrain which sounds can appear next to each other in a language, and morphological rules, when applied blindly, would often violate phonological rules, by resulting in sound sequences that are prohibited in the language in question. For example, to form the plural of dish by simply appending an -s to the end of the word would result in the form *, which is not permitted by the phonotactics of English. In order to "rescue" the word, a vowel sound is inserted between the root and the plural marker, and results. Similar rules apply to the pronunciation of the -s in dogs and cats: it depends on the quality (voiced vs. unvoiced) of the final preceding phoneme.
Lexical morphology is the branch of morphology that deals with the lexicon, which, morphologically conceived, is the collection of lexemes in a language. As such, it concerns itself primarily with word formation: derivation and compounding.
There are three principal approaches to morphology, which each try to capture the distinctions above in different ways. These are,
Note that while the associations indicated between the concepts in each item in that list is very strong, it is not absolute.
In morpheme-based morphology, word forms are analyzed as arrangements of morphemes. A morpheme is defined as the minimal meaningful unit of a language. In a word like independently, we say that the morphemes are in-, depend, -ent, and ly; depend is the root and the other morphemes are, in this case, derivational affixes. In a word like dogs, we say that dog is the root, and that -s is an inflectional morpheme. This way of analyzing word forms as if they were made of morphemes put after each other like beads on a string, is called Item-and-Arrangement.
The morpheme-based approach is the first one that beginners to morphology usually think of, and which laymen tend to find the most obvious. This is so to such an extent that very often beginners think that morphemes are an inevitable, fundamental notion of morphology, and many five minute explanations of morphology are, in fact, five minute explanations of morpheme-based morphology. This is, however, not so. The fundamental idea of morphology is that the words of a language are related to each other by different kinds of rules. Analyzing words as sequences of morphemes is a way of describing these relations, but is not the only way. In actual academic linguistics, morpheme-based morphology certainly has many adherents, but is by no means the dominant approach.
Lexeme-based morphology is (usually) an Item-and-Process approach. Instead of analyzing a word form as a set of morphemes arranged in sequence, a word form is said to be the result of applying rules that alter a word form or stem in order to produce a new one. An inflectional rule takes a stem, changes it as is required by the rule, and outputs a word form; a derivational rule takes a stem, changes it as per its own requirements, and outputs a derived stem; a compounding rule takes word forms, and similarly outputs a compound stem.
Word-based morphology is a (usually) Word-and-paradigm approach. This theory takes paradigms as a central notion. Instead of stating rules to combine morphemes into word forms, or to generate word forms from stems, word-based morphology states generalizations that hold between the forms of inflectional paradigms. The major point behind this approach is that many such generalizations are hard to state with either of the other approaches. The examples are usually drawn from fusional languages, where a given "piece" of a word, which a morpheme-based theory would call an inflectional morpheme, corresponds to a combination of grammatical categories, for example, "third person plural." Morpheme-based theories usually have no problems with this situation, since one just says that a given morpheme has two categories. Item-and-Process theories, on the other hand, often break down in cases like these, because they all too often assume that there will be two separate rules here, one for third person, and the other for plural, but the distinction between them turns out to be artificial. Word-and-Paradigm approaches treat these as whole words that are related to each other by analogical rules. Words can be categorized based on the pattern they fit into. This applies both to existing words and to new ones. Application of a pattern different from the one that has been used historically can give rise to a new word, such as older replacing elder (where older follows the normal pattern of adjectival superlatives) and cows replacing kine (where cows fits the regular pattern of plural formation). While a Word-and-Paradigm approach can explain this easily, other approaches have difficulty with phenomena such as these.
See main article: Morphological typology. In the 19th century, philologists devised a now classic classification of languages according to their morphology. According to this typology, some languages are isolating, and have little to no morphology; others are agglutinative, and their words tend to have lots of easily separable morphemes; while others yet are inflectional or fusional, because their inflectional morphemes are "fused" together. This leads to one bound morpheme conveying multiple pieces of information. The classic example of an isolating language is Chinese; the classic example of an agglutinative language is Turkish; both Latin and Greek are classic examples of fusional languages.
Considering the variability of the world's languages, it becomes clear that this classification is not at all clear cut, and many languages do not neatly fit any one of these types, and some fit in more than one way. A continuum of complex morphology of language may be adapted when considering languages.
The three models of morphology stem from attempts to analyze languages that more or less match different categories in this typology. The Item-and-Arrangement approach fits very naturally with agglutinative languages; while the Item-and-Process and Word-and-Paradigm approaches usually address fusional languages.
The reader should also note that the classical typology mostly applies to inflectional morphology. There is very little fusion going on with word formation. Languages may be classified as synthetic or analytic in their word formation, depending on the preferred way of expressing notions that are not inflectional: either by using word formation (synthetic), or by using syntactic phrases (analytic).
(Abbreviations: CUP = Cambridge: Cambridge University Press; UP = University Press)